1. Field of the Invention
This invention relates to a frame relay apparatus (or frame handler) and a frame relay method to relay information in a communication network in a unit of a frame efficiently in time.
2. Description of the Related Art
"A Semi Store-and-Forward Packet Switching Method" in Published Unexamined Japanese Patent Application Sho 63-224443 and "A Frame Check Sequence Method" in Published Unexamined Japanese Patent Application Sho 62-57345 describe a semi store-and-forward switching technic of a packet or a frame. The packet used in ISDN (Integrated Services Digital Network), as described in the former application, and the frame used in the frame relay method, as described in the latter application, differ both in form and in the layers at which they are handled. However, the packet and the frame are the same in the sense that they treat a group of data as a unit for communication purposes. Therefore, hereinafter the word "frame" is used instead of the word "packet", even though "packet" is originally used in the published unexamined patent application.
FIG. 32 shows a configuration of a conventional semi store-and-forward switching method, employed in order to shorten a total communication time. In FIG. 32, terminals 1a and 1b, frame relay apparatuses (FR apparatuses, hereinafter) 2a and 2b, access lines 3a and 3b and a relay line 4 are shown. The terminal 1a is connected to the FR apparatus 2a through the access line 3a, and tile terminal 1b is connected to the FR apparatus 2b through the access .line 3b. The FR apparatus 2a and the FR apparatus 2b are connected to each other via the relay line 4.
FIG. 33 shows a sample time sequence of a frame transfer in the configuration shown in FIG. 32. In FIG. 33, time is shown on the ordinate. The time sequence of the frame transmitted via the access line 3a, the relay line 4 and the access line 3b is shown respectively in the time sequence 5a, 6 and 5b, A frame 7, a header 8, information 9 and a frame check sequence 10 are also shown. Time t1, t2, t3 and t4 are used later in the explanation. In FIG. 33, the access line 3a and 3b and the relay line 4 are assumed to have the same transmission rate.
At first, a composition of a frame is explained. The frame 7 consists of the header 8, the information 9 and the frame check sequence 10. In the frame relay method, the header 8 includes a logical link number.
In each FR apparatus, a logical link number is provided in advance and is used in the two communicating terminals and in the route connecting the terminals. The logical link number is an independent number in each access line and relay line in the route which connects the terminals. Therefore, the FR apparatus keeps information on which communication line and which logical link number should be used to transmit the Frame 7 when the frame 7 is received via a certain communication line with a certain logical link number. Such information is kept in a table, for example.
Practically, when the FR apparatus receives the frame 7, tile FR apparatus determines the transmitting line and renews the logical link number in the header 8 to a new logical link number for tile next transmitting line according to the received line and the logical link number in the header 8 by referring to the table, for example.
A relay operation is explained chronologically, as follows. In FIG. 33, the terminal 1a starts a transmission of the frame 7 at the time t1. When the whole header of the frame is transmitted, the FR apparatus 2a receives the whole header 8 of the frame 7 at the time t2. The relay line 4 on which the frame 7 is transmitted is decided according to the logical link number included in the header 8 and the logical link number of the header 8 is renewed to a number for the next transmitting line. When the relay line 4 is available for use, the FR apparatus 2a starts a transmission of the received frame 7 around time t2.
In the same way, when the whole header is transmitted at the time t3, the FR apparatus 2b decides the next transmitting line and renews the logical link number of the header 8. In this case, the next transmitting line is tile access line 3b for the terminal 1b. When the access line 3b is also available for use, the FR apparatus 2b starts a transmission of the received frame 7 around time t3. The terminal 1b receives the whole frame 7 at the time t4.
the prior methods, an exclusive error detection signal is provided for the header 8 of the frame 7. By placing the signal either Just before or after the header 8, the frame 7 is prevented from being sent to an unintended destination due to wrong header information, when only the header 8 is received.
As stated above, in the conventional semi store-and-forward switching method, a communication time is shortened. However, an operation is not considered for the case when the rate of the receiving line and the rate of the transmitting line are different.
FIG. 34 shows the time sequence when the transmission rates of the lines are different. In FIG. 34, an abort signal 11 and a time t21 are shown. It is assumed that the configuration is same as FIG. 32, but the transmission rate of the relay line 4 is twice the transmission rate of the access line 3a and 3b.
The operation is as follows. The FR apparatus 2a receives the whole header 8 of the frame 7 from the access line 3a at the time t2. Then, the next transmitting line is decided as the relay line 4 and the header 8 is renewed at once. When the relay line 4 is available for use, the header 8 in the frame 7 starts to be transmitted.
The FR apparatus 2a transmits the information 9 to the relay line 4 continuously after the transmission of the header 8. However, the transmission rate to the relay line 4 is twice that of the receiving rate from the access line 3a. Therefore, the receipt gets behind at the time t21, while the information 9 is transmitted. Consequently, the transmitting data to the relay line 4 runs short. This condition is called a transmission underrun hereinafter.
The FR apparatus 2a transmits the abort signal 11 to notify the receiver of the abortion of the transmission. Consequently, the transmission of the frame 7 is failed.
Since the conventional semi store-and-forward switching method is composed as above mentioned, a communication time is shortened when a frame is relayed between the transmission lines with same communication rates. However, the semi store-and-forward switching method is unable to be applied to a case where the transmission rate of the output signal is higher than the transmission rate of the input signal.
This invention relates to addressing this problem. This invention aims at inventing a frame relay apparatus and a frame relay method with an excellent communication time efficiency, regardless of the transmission rates of the receiving and transmitting lines.